A passive optical network (PON) is a flexible access network that is capable of providing a range of broadband and narrow-band services for business and residential customers. The underlying equipment is considered to be relatively inexpensive for network operators because they do not require any active equipment or power supplies between the operator's central office (CO) and customer's premises (CP). As shown in the PON 10 of FIG. 1, downstream PON traffic is destined from the Optical Line Termination (OLT) 12 residing in the CO towards a number of Optical Network Terminals (ONTs) 16 (or Optical Network Units (ONUs), not shown), residing in the CPs via an optical splitter 14.
Since the OLT 12 is the only unit transmitting in the downstream direction, there can be no collision between downstream-bound packets. Upstream PON traffic shares the same optical fiber with the downstream traffic, utilizing a different wavelength. Therefore, there cannot be any collision between downstream and upstream packets either. However, since the upstream traffic originates from all ONUs 16, and all ONUs are transmitting on the same wavelength, packet collision can occur if two or more ONUs 16 are transmitting simultaneously. In order to prevent collisions, upstream PON traffic is managed in the Time Division Multiple Access (TDMA) fashion. One of the functions of the OLT 12 is to schedule and grant separate time slots to each ONU 16, thus avoiding collision between upstream packets. Transmitter lasers of each ONU 16 can be turned on only during their respective transmission time slots.
The OLT 12 must be capable of receiving bursts of data from different ONUs. A typical burst-mode receiver consists of a photo detector (PD), transimpedance amplifier (TIA), limiting amplifier (LA) and clock and data recovery (CDR) circuitry. The PD performs conversion of the received optical signal into an electrical signal. TIA and LA restore the latter to a standard digital voltage level, whereas the CDR recovers the clock and extracts the transmitted data contents from the LA output signal.
The evolution of PON systems and their underlying standards has seen a steady increase in PON bit rate ranging from the initial 155 Mb/s in APON in the mid-1990s, to 1.25 Gb/s in Gigabit-capable PON (GPON) [ITU-T G.984] and Ethernet PON (EPON) [IEEE 802.3ah] of mid-2000s, up to 10 Gb/s specified in the IEEE 802.3av (10GEPON) [1-3] and ITU-T 10G GPON standards that are currently being drafted. The high bit rates pose an increasing challenge for implementation of the burst-mode receiver, particularly of its analog circuits. It can be difficult to design the TIA and LA that can restore the received signal fast enough and without distortion of its duty cycle, while supporting a wide dynamic range of the input signal. As a result of higher signal distortion at the beginning of upstream frames, the probability of bit errors is typically higher than for other parts of upstream frames. This issue is exacerbated at higher bit rates at which other causes of bit errors, like optical dispersion, are more pronounced.
The frame delimiter is the most significant field of the upstream frame, because the reception of the whole frame depends on the receiver's successful detection of the delimiter. The fact that the delimiter is located at the very beginning of the frame where signal distortion is most likely and that, unlike all other parts of the frame, it is not protected by forward error correction (FEC) error-control code, make the issue of its detection even more critical. In a typical GPON deployment, the overall subscriber packet loss in the upstream traffic is dominated by the loss of frame reception due to failed delimiter detection, for example where the delimiter is too short, or where a detection algorithm either does not accept delimiters with errors or can only handle a limited number of errors. The emerging PON standards such as XGPON [FSAN] and 10G EPON (IEEE 802.3av) are expected to show the same sensitivity related to delimiter detection.
Current solutions for delimiter detection centre around higher tolerance to bit errors. This solution is inadequate because it is not immune to longer bursts of bit errors and because it can lead to false delimiter detection.
What is required is an improved system and method for delimiter detection in a passive optical network.